The effect of font size on reading performance in strabismic amblyopia: an eye movement investigation.

PURPOSE We investigated the effect of font size on reading speed and ocular motor performance in strabismic amblyopes during text reading under monocular and binocular viewing conditions. METHODS Eye movements were recorded at 250 Hz using a head-mounted infrared video eye tracker in 15 strabismic amblyopes and 18 age-matched controls while silently reading paragraphs of text at font sizes equivalent to 1.0 to 0.2 logMAR acuity. Reading under monocular viewing with amblyopic eye/nondominant eye and nonamblyopic/dominant eye was compared to binocular viewing. Mean reading speed; number, amplitude, and direction of saccades; and fixation duration were calculated for each font size and viewing condition. RESULTS Reading speed was significantly slower in amblyopes compared to controls for all font sizes during monocular reading with the amblyopic eye (P = 0.004), but only for smaller font sizes for reading with the nonamblyopic eye (P = 0.045) and binocularly (P = 0.038). The most significant ocular motor change was that strabismic amblyopes made more saccades per line than controls irrespective of font size and viewing conditions (P < 0.05 for all). There was no significant difference in saccadic amplitudes and fixation duration was only significantly longer in strabismic amblyopes when reading smaller fonts with the amblyopic eye viewing. CONCLUSIONS Ocular motor deficits exist in strabismic amblyopes during reading even when reading speeds are normal and when visual acuity is not a limiting factor; that is, when reading larger font sizes with nonamblyopic eye viewing and binocular viewing. This suggests that these abnormalities are not related to crowding.